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How to Make a Strong Complex Factors Effecting M—L Binding Strength = Molecular Organization Complementarity = sum of size, geometry, and electronic matching between the metal ion and the ligand(s) The individual components are simple and can be predicted or found experimentally

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ch12 lecture 2 the chelate effect
How to Make a Strong Complex
    • Factors Effecting M—L Binding Strength = Molecular Organization
      • Complementarity = sum of size, geometry, and electronic matching between the metal ion and the ligand(s)
        • The individual components are simple and can be predicted or found experimentally
        • Example: HSAB Theory predicts Fe3+/O2- is more complementary than Fe3+/S2-
        • Example: d8 Ni2+ should have good complementarity with cyclam
        • Complementarity is only the first step towards complex stability
      • Constraint = the number of and flexibility between ligand donor atoms
        • Topology = interconnectedness of donor atoms
        • Rigidity = how fixed in space donor atoms of the ligand are with respect to each other
        • These constraint factors are more difficult to grasp than complementarity
        • Maximizing these factors can lead to huge increases in complex stability
Ch12 Lecture 2 The Chelate Effect
constraint and binding affinity
Constraint and Binding Affinity

Rigidity

Binding

Affinity

Topology

Electronics

Geometry

Size

Complementarity

Constraint

slide3
Topological Effects
    • The Chelate Effect
      • Two donor atoms linked together = a chelate (claw)
      • Chelate ligands form much more stable metal complexes than monodentate related ligands (up to 105 times as stable)

Ni2+ + L Formation Constants:

L = NH3 en trien 2,3,2

log b 8.12 13.54 13.8 16.4

      • Thermodynamic Reasons for the Chelate Effect = Entropy

better

complementarity

slide4
Kinetics and the Chelate Effect
    • Chelate complex formation
slide5
The Steady-State Approximation yields:

Or rewriting with formation (kf) and dissociation (kd) constants:

  • Assuming a chelate effect, k2 >>k-1
  • kf is not the source of the chelate effect. It is the same as other ligands
slide6
v. kd must be the source of the chelate effect (dissociation is slow!)

k-1 is the same as for monodentate ligands

k-2 (ring opening) is the same as for monodentate ligands:

Data for k2 (ring closing)

Huge Concentration!

slide7
k2, the formation of the second M—L bond, has been shown to be extremely large compared to a second monodentate ligand binding. This is due to the large “effective concentration” of the second donor atom of a chelate

If k2 is large, kd must be small;

Very fast bond reformation after the first donor dissociates is the kinetic source of the chelate effect

vi. Data

slide8
The Macrocycle Effect
    • Macrocyclic chelate complexes are up 107 times more stable than non-cyclic chelates with the same number of donors

Ni(trien)2+ + H+ Ni2+ + H4trien4+ t½ = 2 seconds

Ni(cyclam)2+ + H+ Ni2+ + H4cyclam4+ t½ = 2 years

    • Connecting all of the donors (having no end group) makes k-2 important
      • Breaking the first M—L bond requires major ligand deformation
      • The increase in Ea required greatly slows down k-2
    • A macrocycle is still a chelate, so it still has the k2 chelate effect going
    • The result is a very stable complex as kd becomes miniscule
slide9
The Cryptate Effect
    • Additional connections between donor atoms in a macrocycle further enhance complex stability by making dissociation even more difficult
    • My own research and others’ seek to take advantage of this stability
    • Data

Cu(H2O)62+ water substitution t½ = 1.4 x 10-9 s

Cu(Me4Cyclam) in 1 M H+ t½ = 2 s

Cu(Bcyclam)Cl+ in 1 M H+ t½ > 6 years = 1.9 x 108 s

    • Usefulness of such stable complexes
      • Oxidation catalysis in harsh aqueous conditions (H+ or OH-)
      • MRI Contrast agents that must not dissociate toxic Gd3+
slide10
Rigidity Effects
    • More rigid ligands (assuming complementarity) make more stable complexes
    • Data
slide11
Encompassing Principles
      • Multiple Juxtapositional Fixedness (Busch, 1970) = lack of end groups and rigidity effects leads to more stable complexes for topologically complex ligands if complementarity is satisfied
      • Pre-Organization (Cram, 1984): Ligands pre-formed into size and geometry match for the metal ion do not require entropically costly reorganization to bind. This savings in entropy leads to more stable complexes

Problem: Too much pre-organization can make it hard to get the metal in!

  • Stereochemistry of Reactions
    • Retention or Inversion of Stereochemistry is possible during substitution reactions
      • Retention is usually favored for D and ID mechanisms
      • Inversion is often the result of the SN1CB mechanism
      • The orientation of the ligand entering the trigonal bipyramidal intermediate determines the outcome
slide13
Substitution in trans complexes

1) 3 possible substitution reactions for trans-[M(LL)2BX] + Y

a) Retention of configuration with

a square pyramidal intermediate

b) Trigonal bipyramidal intermediate

with B in the plane gives a mixture

of products

c) Trigonal bipyramidal intermediate

with B axial leads to cis product

slide14
Experimental Data
      • Many factors determine the mixture of isomers in the product
      • Example: Identity of X
      • Prediction is very difficult without experimental data on related complexes
  • Substitution in cis complexes
    • The same 3 possibilities exist as for trans
    • The products are just as hard to predict
slide16
Isomerization of Chelate Complexes
    • One mechanism is simple dissociation and reattachment of one donor of the ligand. This would be identical to any other substitution reaction
    • Pseudorotation
      • “Bailar Twist” = Trigonal twist = all three rings move together through a parallel intermediate
      • Tetragonal Twists = one ring stays the same and the others move

Bailar Twist

Tetragonal Twist

Tetragonal Twist

Bailar Twist